1. Field of the Invention
The present invention relates to an extreme ultra violet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation.
Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than several tens of watts.
FIG. 10 is a diagram for explanation of a principle of generating EUV light in an LPP type EUV light source apparatus. The EUV light source apparatus shown in FIG. 10 includes a driver laser 901, a collective lens 902, a target supply unit 903, a target supply nozzle 904, and an EUV collector mirror 905. The driver laser 901 is a laser light source for generating a laser beam for exciting a target material by pulse oscillation. The collective lens 902 collects the laser beam outputted from the driver laser 901 in a predetermined position. The target supply unit 903 supplies the target material to the target supply nozzle 904 and the target supply nozzle 904 supplies the target material to the predetermined position.
When the laser beam is applied to the target material supplied from the target nozzle 904, the target material is excited and plasma is generated, and light having various wavelength components is radiated from the plasma. The EUV collector mirror 905 has a concave reflection surface that reflects and collects the light radiated from the plasma. On the reflection surface, a film in which molybdenum and silicon are alternately stacked (Mo/Si multilayer film), for example, is formed for selective reflection of a predetermined wavelength component (e.g., near 13.5 nm). Thereby, the EUV light having the predetermined wavelength component radiated from the plasma is outputted to an exposure unit or the like.
In the LPP type EUV light source apparatus, there is a problem of the influence by charged particles such as fast ions emitted from plasma. This is because the EUV collector mirror 905 is located near the emission point, and thus, the fast ions and so on collide with the EUV collector mirror 905 and the reflection surface (Mo/Si multilayer film) of the mirror is sputtered and damaged. In order to improve the EUV light use efficiency, it is necessary to keep the reflectance of the EUV collector mirror 905 high. For the purpose, high flatness is required for the reflection surface of the EUV collector mirror 905, and the mirror becomes very expensive. Accordingly, longer life of the EUV collector mirror 905 is desired in view of reduction in operation cost of the exposure equipment using the EUV light source apparatus, reduction in maintenance time, and so on.
As a related technology, Japanese Patent Application Publication JP-P2005-197456A discloses a light source apparatus in which the life of the collector mirror can be extended and the running cost can be reduced by protecting the collector mirror from debris that may cause damage to the mirror coating while securing the catching solid angle and the catching efficiency of the EUV light. The light source apparatus includes a target supply unit for supplying a material to become a target, a laser unit for applying a laser beam to the target to generate plasma, a collective optics for collecting and outputting extreme ultra violet light emitted from the plasma, and magnetic field generating means for generating a magnetic field within the collective optics to trap charged particles emitted from the plasma when current is supplied. As the magnetic field generating means, a pair of mirror coils with the collective optics in between or a baseball coil accommodating the collective optics is used.
Further, U.S. Pat. No. 6,987,279 B2 discloses a light source device including a target supply unit for supplying a material to become the target, a laser unit for generating plasma by applying a laser beam to the target, a collection optical system for collecting the extreme ultra violet light radiating from the plasma and emitting the extreme ultra violet light, and magnetic field generating unit for generating a magnetic field within the collection optical system when supplied with current so as to trap charged particles radiating from the plasma. In the light source device, ions radiating from the plasma are trapped near the plasma by forming a mirror magnetic field by using Helmholtz electromagnets. Thereby, the damage on the EUV collector mirror by so-called debris of ions and so on is prevented.
In the EUV light source apparatus using two coils as disclosed in JP-P2005-197456A, the two coils are arranged to be in parallel or substantially in parallel with each other and opposed such that the centers of the apertures of the two coils are aligned. The EUV collector mirror is provided in a gap space between the two coils, and accordingly, sufficient magnetic field intensity is necessary in the gap space between the two coils. The necessary magnetic field intensity differs depending on the distance from the coil to the EUV collector mirror, the mass number, charge states, kinetic energy of ions to be generated, and so on, and it is considered that the higher the magnetic field intensity, the more desirable. Since the gap length between the two coils and the center magnetic field intensity have an inverse relationship, the shorter the gap, the easier the generation of a strong magnetic field, while the longer the gap, the harder the generation of the strong magnetic field.
The currently envisioned diameter of the EUV collector mirror used in the EUV light source apparatus is about 300 mm, and a gap length of 300 mm or more is necessary. Further, in view of various mechanisms necessary for the EUV light source apparatus such as a holding and adjustment mechanism of the EUV collector mirror and a holding and adjustment mechanism of the laser beam focusing optics, it is necessary to make the gap length longer. For example, according to the technical estimation of an existing superconducting magnet, in the case of a conventional mirror magnetic field, the center magnetic field is 3 T at the maximum in a gap of 450 mm, while the maximum magnetic field becomes lower to 2.3 T in a gap of 600 mm. Therefore, in the case of using two coils, for generation of a stronger magnetic field, it is necessary to arrange the two coils at the possible minimum distance with the EUV collector mirror in between.
Consequently, it is considered that the two coils are provided within a vacuum chamber in which plasma is generated. In this case, in order to keep the degree of vacuum within the chamber and prevent emission of contaminants, various ideas are necessary for the respective coils to isolate the coils from the vacuum space within the chamber. Further, in the case of using two coils, very strong force acts on the coils attracting (or repelling) each other due to the magnetic field, and a strong structure (support) for counteracting the force is necessary. Therefore, in the case of using two coils, a large structure (support) is necessary between the two coils and the available space is limited.
In the case of using a pair of mirror coils as shown in FIG. 3 of JP-P2005-197456A, a magnetic field distribution called a mirror magnetic field is obtained, in which the magnetic flux density increases vertically symmetrically in the Z-axis direction from the plasma position. In the case of such a symmetric magnetic field, the main purpose is to prevent damage on the collector mirror by confining charged particles in the valley of potential by the mirror magnetic field generated by the coils 6 and 7 and trapping them around the emission point. It is considered that, when the charged particles are trapped around the emitting point in this manner, the concentration of the target gas (ions and neutralized atoms of the target material) remaining within the vacuum chamber rises. The target gas absorbs the EUV light radiated from the plasma, and accordingly, there occurs a problem that the available EUV light is reduced because of the concentration rise. Further, when a metal or the like is used as the target, there is concern that the remaining target gas attaches to the structures within the chamber and the EUV collector mirror and so on. Especially, in the case of plasma generation by highly-repeated operation required for the EUV light source apparatus, it is considered that such a tendency is remarkable.
Further, in the case of using a baseball coil as shown in FIG. 7 of JP-P2005-197456A, the magnetic flux density increases in all directions from the center of the magnetic field, and accordingly, a magnetic field distribution for the purpose of complete trapping (confining) is generated, and the same problems occur. For the above-mentioned reasons, it is desirable that the charged particles generated in the plasma are efficiently and promptly ejected without spending time rather than trapped or ejected in a long time.
In terms of behavior stability of charged particles, in the case of the mirror magnetic field, the magnetic field intensity in a direction orthogonal to the Z-axis becomes weaker toward the outside, and therefore, fast charged particles having high energy behaves very unreliably so as to twine around the lines of magnetic force while flying out, and the ejection of charged particles is not very good. On the other hand, in the case of the baseball magnetic field, the symmetric magnetic field distribution in which the magnetic field intensity increases from the center toward all directions, and therefore, the behavior stability of charged particles is high in trapping. However, the original purpose of the baseball magnetic field is trapping, and the ejection of charged particles is very poor.
Further, in U.S. Pat. No. 6,987,279 B2, in order to efficiently eject ions and so on from the vicinity of the plasma and the collector mirror to reduce the concentration of the target gas (ions and neutralized atoms of the target material) remaining near the plasma, the magnetic field is formed such that the magnetic flux density becomes lower on the opposite side of the collector mirror. By the influence of the magnetic field, the ions and so on are guided toward the lower magnetic flux density, i.e., toward the opposite direction to the collector mirror.
However, even when the ions and soon can be guided outside of the magnetic field in this manner, the concentration of the target gas remaining within the chamber rises again unless the ions and so on are efficiently ejected to the outside of the chamber. The target gas absorbs the EUV light radiated from the plasma, and accordingly, there occurs a problem that the available EUV light is reduced because of the concentration rise. Therefore, it is necessary to provide a mechanism for efficiently ejecting the target gas to the outside of the chamber (e.g., an eject opening having a large diameter) in an appropriate position in addition to the configurations shown in FIGS. 6A and 7 of U.S. Pat. No. 6,987,279 B2.
Providing an ejection mechanism of ions and so on in the devices shown in FIGS. 6A and 7 of U.S. Pat. No. 6,987,279 B2 causes the following problems. That is, in a general EUV light source apparatus, a filter for purifying the spectrum of the EUV light, a coupling mechanism to an exposure unit, and so on are provided at the side opposite to the EUV collector mirror (the traveling direction of the reflected EUV light). Accordingly, in consideration of the interference with the filter, coupling mechanism, and so on, it is difficult to provide the ejection mechanism of ions and so on at the side opposite to the EUV collector mirror. However, when the ejection mechanism, particularly, the position of the ejection opening formed in the chamber is inappropriate, the ejection speed of ions and so on becomes lower and the concentration of ions and so on rises within the chamber. Especially, in the case of EUV light generation by highly-repeated operation, it is considered that such a tendency is remarkable.